A light-emitting element including a layer containing a light-emitting material between a pair of electrodes is used as a pixel, a light source, or the like and is provided in a light-emitting device such as a display device or a lighting device. Upon applying current between the pair of electrodes of the light-emitting element, an excited light-emitting material emits fluorescence or phosphorescence.
In theory, internal quantum efficiency of phosphorescence is three times as large as that of fluorescence in the case of current excitation by comparing phosphorescence with fluorescence. Therefore, it is considered that luminous efficiency is increased by using a light-emitting material which emits phosphorescence instead of using a light-emitting material which emits fluorescence, and so a material which emit phosphorescence has been developed.
For example, a patent document 1 discloses a metal complex having iridium as its central metal. In accordance with the patent document 1, the metal complex can be used as a light-emitting material.
Patent document 1: Unexamined patent publication No. 2001-247859